A wide variety of minerals are best recovered from their underground deposits by what is called solution mining, a process in which steam, hot water or cool water is injected into the mineral bed in a first well, and a mineral-laden brine is pumped up a second well.
An example of solution mining of nahcolite is shown in U.S. Pat. No. 4,815,790, of which I am one of the co-inventors. That patent shows the use of hot water under special conditions of pressure and temperature to recover a brine of nahcolite, a sodium bicarbonate mineral. In the background of that patent are discussed some of the problems of prior art solution mining techniques, for example the formation of "morning glory" holes which are generally narrow at the base and flare outward at the top in a generally convex upward cross-sectional floor profile. A variety of techniques have been attempted in order to prevent the formation of such types of holes, since they are very wasteful and since they result in a low percentage of mineral recovery from the bed. One of these techniques involves use of an air cushion above the level of the fluid in the cavity to achieve a more or less cylindrical solution cavity.
Solution mining of salt to obtain a saturated brine is known to have been used in France as early as A.D. 858, and is the basis of present technology. Solution mining of salt was first employed in the United States in about 1882, and consisted of drilling a small diameter well down to a layer of salt, pumping freshwater down to dissolve the salt, and pumping the resultant brine to the surface for subsequent evaporation. One of the more modern solution mining techniques is where a first injection well is sunk, and pressurized freshwater is introduced to hydraulically fracture the bedded salt. Once communication with a second, laterally positioned production well is established, the brine is pumped to the surface for treatment.
Roof collapse of the overlying strata and surface subsidence are potential problems associated with solution mining; however, some precautions can be taken to minimize these hazards. One method is to inject air with the water into the salt caverns. The air forms a protective cover between the water and the top of the cavity and thereby reduces the amount of dissolution of the roof.
Several U.S. patents have been devoted to either solution mining or jet cutting. For example, Cannon U.S. Pat. No. 3,311,411 discloses mining of a granular water insoluble (as distinct from a monolithic bedded) phosphate ore by use of a down-well positive displacement pump at the lower end of a vertical conduit sealed in the well. The method depends on inducing lateral flow of the granular ore to the casing by the suction of the pump.
Claytor U.S. Pat. No. 1,851,565 mines oil-bearing sands or sulfur using a heated solution of sodium carbonate projected laterally from a vertical string through a hinged side arm nozzle, the purpose being to fluidize the oil or melt the sulfur. A second, downwardly directed nozzle agitates the area directly below the vertical pipe to provide a sump for the production pipe inlet at the bottom of the string. A lifting arm raises the arm from a vertically downward position (which permits it to be lowered downwell) to a horizontal position. The combination of undercutting plus solution mining is not taught or suggested as Claytor is directed to use of the hinged nozzle to fluidize the entire tar sands or sulfur bed.
Wenneborg U.S. Pat. No. 3,747,696 slurry mines granular water insoluble phosphate ore by use of a vertical drill string having a directionally indexable but non-rotable section bearing a sideward directed jet, the opening and closing of which can be controlled by hydraulic pressure acting on a valve/lever assembly. The method shown in Wenneborg's FIG. 2 apparently top cuts, as compared to undercutting, and the resulting cylindrical cavern has a floor sloping downward to the center. Solution mining is not taught.
Fly U.S. Pat. No. 3,155,177 is directed to a vertical, hydraulically powered cutter and pump which is rotated from the surface and has horizontally directed cutting jets, which are movable up or down or controllably rotated, with a hydraulic jet pump located therebelow in a submerged sump. Opposed side wall jets are used to cancel reaction thrust to insure the drill string hangs vertically in the well. Liquid hydrocarbons are used as the hydraulic cutting fluid to under-ream tar sands. The side wall cutters are said to be useable without rotation to form pairs of lateral trenches. A series of vertical holes would permit forming an interconnected tunnel with adjacent trench floors forming a series of interconnected V's. Solution mining is not shown and it is not clear whether the cutting proceeds top down or bottom up. In any event, the sloping bottoms are not indicative of undercutting.
The mining and processing of rock salt can bring about a degree of disruption to local environments and existing ecological systems. A major environmental concern in solution mining of salt is land subsidence. As the salt is dissolved, some roof collapse may occur, causing sections of the surface to partially or totally fill the cavity. Subsidence is unpredictable, and once the process begins, it must be allowed to finish and reach equilibrium.
The world resources of salt are virtually unlimited. The identified salt resources of the United States alone are estimated as 61.times.10.sup.12 short tons (st). World salt production estimates by the Bureau of Mines rank the US first with 34 Mt (37.5 million st), followed by China, 18 Mt (20 million st); the Soviet Union, 16 Mt (18 million st); the Federal Republic of Germany, 13.6 Mt (15 million st); India, 11.2 Mt (12.4 million st); and Canada, 9 Mt (10 million st). Other major producers are France, the United Kingdom, Australia, Poland, and Mexico. Total world salt production in 1988 was 179 Mt (197 million st).
The production of potash in the United States is declining as lower ore grades are being mined, reserves are being depleted, and new economic deposits have yet to be discovered. Mining lower ore grades results in higher costs per ton of product at the mine and leads to a small marketing area when transportation costs are added. As a result of this decline, the United States is becoming increasingly dependent upon potash imports from Canada.
Estimated domestic potash resources total about 6 billion Mt K.sub.2 O equivalent. Most of this lies between 1,800 and 3,000 meters deep, in a 3,100 square kilometer area of Montana/North Dakota as an extension of the Williston Basin Deposits in Saskatchewan. The Paradox Basin in Utah contains approximately 2 billion Mt K.sub.2 O, mostly at depths more than 1,200 meters. An unknown quantity of potash resources lie about 2,100 meters deep under central Michigan. These resources can be extracted only by solution-mining techniques because of the bed depth. Operation of a solution mine in Saskatchewan for several years has demonstrated the commercial viability of solution mining under certain conditions. Extensive potash occurrences in the form of polyhalite in west Texas and New Mexico are not included because current technology does not permit economic recovery of this mineral.
For example, the Cane Creek Syncline Mine (Texas Gulf mine) near Moab, Utah, was converted to a solution mine after 6 years of underground mining because much folding was encountered, along with methane gas. The single solution mine at Belle Plaine (the Kalium Mine) in Saskatchewan, Canada, was originally developed as a solution mine because the ore zone was below the reasonable depth (3,500 feet) for underground mining in a sedimentary sequence.
In Michigan, Dow Chemical Co. core-sampled bedded sylvinite near Midland, and evidence was obtained that potash may underlie some 33,700 square kilometers of the Michigan Basin. The potash occurs in a stratigraphic unit known as the A-1 Salt of the Salina Group, of The Silurian Period. At Midland, the salt layer containing the potash is 120 meters thick and is at a depth of about 2,440 meters. Kalium Chemicals of PPG Industries, Inc., is strongly considering solution mining potash west of Midland, between Big Rapids and Reed City. Limited released data indicates the enriched zones/beds of potash within the A-1 salt vary in thickness from a few centimeters to approximately ten meters or more and with ore grades varying from 2% to 64% KCl.
While solution mining of sylvinite may bring about the reclassification of the Michigan deposit from the resource to the reserve category, even so the United States is expected to continue to be a net importer of potash.
Sylvinite ore can be mined by injecting water through a well and withdrawing a NaCl-KCl saturated solution through another well, or by using concentric pipes in a single well. To control the shape of the solution cavity, the solution can be blanketed by a layer of oil or gas at the roof. Solution mining can be considered if the beds are very irregular or if they are at depths greater than 1,100 meters where halite creep becomes a problem, but the ore zones have to be thicker than about 15 meters or included for solution mining to work under the current practices.
World potash demand increased in 1987-88 for the second year in a row, reaching a record level of 27.6 Mt (30 million st) K.sub.2 O. This was an increase of 5.2% over the 1986-87 demand of 26 Mt (29 million st). The grown in consumption was the net result of a 22.7% increase in demand in developing countries (1.2 Mt or 1.3 million st). Total world potash production increased by 1.6 Mt (1.8 million st) K.sub.2 O in 1987-88 to 30.4 Mt (33.5 million st) in response to the higher market demand. World potash demand is expected to grow 1.5% to 2% per year for the next decade as developing countries strive to increase crop production to feed their growing populations and reduce the cost of imported foodstuffs. The FAO/World Bank/UNIDO Industry Working Group on Fertilizers forecasts an increase in potash consumption from 27.6 Mt (30 million st) K.sub.2 O in 1987/88 up to 31.7 Mt (35 million st) in 1997/98. Production in the U.S. declined by 200 kt (220,000 st), reflecting a reduction in ore grade at some of the older mines. It is estimated that in 1989 domestic mine production will be 1.5 million tons and that the U.S. apparent consumption will be 5.6 million tons.
There are over 60 identified natural sodium carbonate deposits in the world, the largest of which is the trona deposit in southwest Wyoming. The Wilkins Peak Member in the Green River Formation contains 42 beds of trona, 25 of which have a thickness of 3 feet or more. Eleven of these beds exceed 6 feet in thickness and underlie a surface area of more than 1,100 square miles.
Underground mining of Wyoming trona is similar to coal mining, except that trona is a harder mineral than coal. The present Wyoming soda ash producers use room-and-pillar, longwall, shortwall, and solution mining techniques individually or in combination.
FMC has pioneered the use of solution mining to dissolve and recover deeply buried trona. Using an array of injection and recovery wells, a solvent, presumably dilute sodium or calcium hydroxide, is introduced under pressure to dissolve the underlying trona. This technique, although proven, is still in the experimental phase.
Two potential sources of soda ash, nahcolite (sodium bicarbonate) and dawsonite (sodium-aluminum carbonate), are associated with oil shale in the Piceance Creek Basin of northwest Colorado. Identified resources of 32 billion tons of nahcolite and 19 billion tons of dawsonite, equivalent to 20 billion tons and 7 billion tons, respectively, of sodium bicarbonate resources, would be available as a byproduct of oil shale processing or as a single mineral extraction.
In 1988, domestic soda ash production reached a record 8.7 Mt (9.6 million st), an increase of 8% over 1987. Export sales also set a record with total shipments exceeding 2.2 Mt (2.4 million st). These increases were attributed to a rise in domestic and foreign demand for consumer products that use soda ash. A cyclic opportunity also presented itself to sell soda ash to certain crossover markets that traditionally use caustic soda, such as pulp and paper, chemicals, and alumina refining. Apparent consumption of soda ash in the United States rose 7% to 6.7 Mt (7.4 million st).
However, solution mining works best in thickly horizontal beds. One of the problems in mining some types of evaporite minerals, such as, for example, nahcolite, is that the beds may be relatively thin, on the order of a few inches to a few feet. Only occasionally are there beds that range thicker than 15-20 feet. Usually, the thicker the bed the lower the grade of mineral, as it is interspersed with other types of rock deposits, such as in the case of nahcolitic kerogen-bearing rocks. Upon the application of steam, the kerogen rock releases oil which either leaches out or forms an oily froth which interferes with production or quality of mineral sought to be dissolved by the mining solution.
Further, mining of these types of minerals is often hindered by the fact that they may lie in relatively soft overbearing strata. The soluble minerals themselves may actually be somewhat stronger than the softer overlying rocks, which can result in pillars punching holes through the roof, roof collapse, and the like unless the caverns are kept small or morning glory hole shapes (in the case of solution mining) are avoided. All of these necessitate mining smaller cavities with larger support pillars. In the case of room and pillar mining, the use of extensive roof bolting or other shoring techniques normally would be required. Further it is not economically feasible in most situations to room and pillar mine thin beds, even in the case of highly valuable nahcolite mineral.
Nahcolite is an extremely valuable mineral, being used as an air pollution control sorbent. The sodium bicarbonate content reacts with SO.sub.x and NO.sub.x in flue gases of power plants to remove these pollutants. The resulting sodium sulfate wastes may be safely disposed by a variety of techniques such as shown in U.S. Pat. Nos.: 4,726,710 (Co-Disposal I); 4,946,311 (Co-Disposal II); 3,962,080 (Sinterna process); and 3,984,312 (Fersona process).
Accordingly, there is a need to improve solution mining productivity, particularly for evaporites in thin beds, in steeply dipping beds, or in massive deposits. Of particular need is to recover nahcolite present in thin beds in the Piceance Creek Basin in Northwestern Colorado. Being able to control the shape of the cavities, and to solution mine thin, multiple high-grade beds of purity in excess of 60-85% will help make this mineral more available at a lower cost, and thus help solve the nation's air pollution problems, particularly the SO.sub.x g/NO.sub.x emissions from power and industrial plants.